Electrostatic printing



United States Patent ELECTROSTATIC PRINTING Virgil E. Straughan, Euclid,Ohio, assignor, by mesne assignments, to General Dynamics Corporation,San Diego, Calif., a corporation of Delaware No Drawing. ApplicationNovember 3, 1955 Serial No. 544,815

4 Claims. (Cl. 117 38) This invention relates to electrostatic printing.More particularly, it relates to an improvement in the manner in which alatent electrostatic image is converted into a permanent visual recordin an electrostatic printing procedure in which a liquid is employed forthe development of a visual image of the latent electrostatic image.

Processes for forming a latent electrostatic image existing as anelectrostatic charge pattern in a layer of material having a highresistance and for subsequently converting the latent electrostaticimage into a visual image are known. One such process involves the useof a finely divided opaque solid in the form of a suspension in air asdescribed in United States Patent 2,221,776. In the processes describedin copending patent applications Serial No. 380,285, filed September 16,1953, Serial No. 484,215, filed January 26, 1955, and Serial No.486,995, filed February 8, 1955, all of which are assigned to the sameassignee as the instant application, another type of developmentcomprising bringing the surface bearing a latent electrostatic imageinto contact with a body of liquid developer, either byimmersion of theimage bearing surface or by flowing the liquid developer over thesurface is described. In this method, a developer comprising a finelydivided opaque solid dispersed in a liquid having a high electricalresistance is employed.

The advantages of liquid developers are described by Metcalfe in theJournal of Scientific Instruments published by the Institute of Physics(London), volume 32, Number 2, February 1955, pages 74 and 75, and arestated to be greater simplicity and speed of operation coupled withimproved control and versatility in the development process, and theyare said to be especially valuable when an immediate visual inspectionof the image is desired and the image need only persist for a shorttime.

The present invention constitutes an improvement in the manner in whichthe visual image is converted into a permanent record image. I havefound that when attempts were made to transfer the visual image to anabsorbent medium such as newsprint or other paper, the suspending liquidwas often readily absorbed by the transfer medium without transporting asufiicient quantity of the finely divided opaque powder to produce highcontrast. Less absorbent record media were tried, such as an adhesivecoated sheet. To transfer the visual image, pressure was exerted on thepermanent record medium at the point, line, or plane in contact with thevisual image, in an effort to press the finely divided opaque particlesinto the adhesive, whereby they would become permanently affixed. Thisapproach was not entirelysatisfactory since the suspending liquid wasnot absorbed by veloper, and which is adapted, in addition, to produce apermanent visual image from the visual image first obtained.

In accordance with my invention the visual image produced by developingthe latent electrostatic image with a liquid developer is firstcontacted by an absorbent means whereby a major fraction of thesuspending liquid is separated from the visual image and then the visualimage is contacted with a thermoplastic material which has been softenedby heat, and the thermoplastic material then takes up a substantialamount of the finely divided opaque powder forming the dried visualimage and, after the thermoplastic material has cooled'and set, itretains the opaque powder image in the form of a permane'nt visualrecord.

One liquid developer found to be economical, relatively stable and onewhich is simply prepared from readily available commercial productscomprises a suitable concentration of a finely dividedopaque particulatematerial, preferably between 0.01 micron and 50 microns in size in ahigh resistance liquid in which the particlesmay be dispersed and withwhich they do not react to any appreciable extent. One such liquiddeveloper, preferred by me, consists of between 5 and 25% by weight ofpowdered charcoal of 1 micron average particle size suspended inkerosene as the high resistance liquid. Other opaque solid particles andother high resistance liquids may constitute the liquid developerwithout departing from the invention herein disclosed.

In operation, the high resistance liquid carries the opaquepowderparticles to the device bearing a latent electrostatic chargepattern and the opaque particles are deposited, along with' accompanyingliquid, on portions of the charge pattern as is well known, to producean impermanent visual i'mage. I I y In order that the resulting visualimage may be conve'rted into a permanent record image, in accordancewith my invention, the visual image is first contacted with an absorbentmeans. I have found that ordinary newsprint possesses the desiredabsorbency, and even though as much as 20% of the' carbon particlescomprising the visual image may be transferred during the blotting step,the

or it may take the form of a porous roller designed to squeeze offexcess liquid. When the latent electrostatic image is developed into animpermanent visual image on a flat plate-like element, the absorbingmeans may take the form of a sheet blotter or a blotting roller passedover the surface. Whatever the form taken by the absorbing means, itmerely functions to remove as much of the suspending liquid as possiblefrom the impermanent visual image while removing as little as possibleof the opaque powder constituting the image forming means. Once theimpermanent visual image has been suitably conditioned for transfer toform a permanent record, it is contacted by the medium on which thepermanent record is to be formed. Two approaches have been foundsuitable. In the first, a paper coated or impregnated with athermosoftening material such as polyethylene was heated sufficiently tosoften the polyethylene, and then was me-= chanically pressed againstthe image. When the thermo-. softening material hardened it fixed thecarbon or other record the suitably preparedblottedvisual image wasbrought into contact with adhesive covered transfer media such as paperor transparent films. The adhesive removed the opaque powder from themeans supporting it and the visual image was then imprisoned in theadhesive itself.

I claim:

1. In the method of converting a latent electrostatic image into apermanent visual record in an electrostatic printing process whichincludes forming a latent electrostatic charge pattern in a highresistance material and developing a visual image of said charge patternby bringing the surface of said high resistance material into contactwith a developer comprising a solid opaque particulate materialdispersed in a liquid possessing a high electrical resistance therebyestablishing an impermanent visual image on the surface of said highresistance material, said image consisting of deposited solid particleselectrostatically attracted to and held by the latent electrostaticcharge pattern and a film of said liquid in excess of that permittingtransfer of the solid particles to a non-absorbent permanent recordmedium, the improvement which comprises: bringing a porous solidabsorbent member, adapted to absorb liquid, into physical contact withthe film of liquid on the surface of the high resistance materialbearing the impermanent visual image consisting ofeleetrostatically-held solid particles and the film of liquid associatedtherewith, thereby absorbing a substantial portion of the liquid and aminor proportion of the solid particles While leaving a substantiallyliquid-free image formed of the remainder of the solid particles andthereafter transferring the liquid-free image formed of solid particlesto a non-absorbent record medium.

2. The method of claim 1 wherein the developer consists of a dispersionin kerosene of between 5% and 25% by weight of solid charcoal particleshaving a particle size between 0.01 micron and 50 microns.

3. In the method of converting a latent electrostatic image into apermanent visual record in an electrostatic printing process whichincludes forming a latent electrostatic charge pattern in a highresistance material and developing a visual image of said charge patternby bringing the surface of said high resistance material into contactwith a developer comprising a solid opaque material dispersed in aliquid possessing a high electrical resistance thereby establishing animpermanent visual image on the surface of said high resistancematerial, siad image consisting of deposited solid particleselectrostatically attracted to and held by the latent electrostaticcharge pattern and a film of said liquid in excess of that permittingtransfer of the solid particles to a non-absorbent pennanent recordmedium, the improvement which comprises: bringing a porous solidabsorbent member, adapted to absorb liquid, into physical contact withthe film of liquid on the surface of the high resistance materialbearing the impermanent visual image consisting of electrostaticallyheldsolid particles and the film of liquid associated therewith, therebyabsorbing a substantial portion of the liquid and a minor proportion ofthe solid particles while leaving a substantially liquid-free imageformed of the remainder of the solid particles, preparing a permanentrecord medium comprising a paper coated with a thermosoftening materialby heating sufficiently to soften the said material and thereaftertransferring the liquid-free image formed of solid particles to theheated, softened thermosoftening coating.

4. In the method of converting a latent electrostatic image into apermanent visual record in an electrostatic printing process whichincludes forming a latent electrostatic charge pattern in a highresistance material and developing a visual image of said charge patternby bringing the surface of said high resistance material into contactwith a developer comprising a solid opaque particulate materialdispersed in a liquid possessing a high electrical resistance therebyestablishing an impermanent visual image on the surface of said highresistance material, said image consisting of deposited solid particleselectrostatically attracted to and held by the latent electrostaticcharge pattern and a film of said liquid in excess of that permittingtransfer of the solid particles to a non-absorbent permanent recordmedium, the improvement which comprises: bringing a porous solidabsorbent member, adapted to absorb liquid, into physical contact withthe film of liquid on the surface of the high resistance materialbearing the impermanent visual image consisting ofelectrostatically-held solid particles and the film of liquid associatedtherewith, thereby absorbing a substantial portion of the liquid and aminor proportion of the solid particles while leaving a substantiallyliquidfree image formed of the remainder of the solid particles andthereafter transferring the liquid-free image formed of solid particlesto a permanent record medium comprising a heated polyethylene paper.

References Cited in the file of this patent UNITED STATES PATENTS2,221,776 Carlson Nov. 19, 1940 2,576,320 Vlad et al NOV. 27, 19512,633,796 Pethick Apr. 7, 1953 2,647,464- Ebert Aug. 4, 1953 2,784,109Walkup Mar. 5, 1957 OTHER REFERENCES Atkinson et al.: Ferrography,Journal of the Franklin Institute, volume 252, No. 5, November 1951,pages 373 to 377.

Metcalfe: Liquid Developer for Xerography, Journal of ScientificInstruments, volume 32, February 1955, pages 74 to 75.

1. IN THE METHOD OF CONVERTING A LATENT ELECTROSTATIC IMAGE INTO APERMANENT VISUAL RECORD IN AN ELECTROSTATIC PRINTING PROCESS WHICHINCLUDES FORMING A LATENT ELECTROSTATIC CHARGE PATTERN IN A HIGHRESISTANCE MATERIAL AND DEVELOPING A VISUAL IMAGE OF SAID CHARGE PATTERNBY BRINGING THE SURFACE OF SAID HIGH RESISTANCE MATERIAL INTO CONTACTWITH A DEVELOPER COMPRISING A SOLID OPAQUE PARTICULATE MATERIALDISPERSED IN A LIQUID POSSESSING A HIGH ELECTRICAL RESISTANCE THEREBYESTABISHING AN IMPERMANENT VISUALL IMAGE ON THE SURFACE OF SAID HIGHRESISTANCE MATERIAL, SAID IMAGE CONSISTING OF DEPOSITED SOLID PARTICLESELECTROSTATICALLY ATTRACTED TO AND HELD BY THE LATENT ELECTROSTATICCHARGE PATTERN AND A FILM OF SAID LIQUID IN EXCESS OF THAT PERMITTINGTRANSFER OF THE SOLID PARTICLES TO A NON-ABSORBENT PERMANENT RECORDMEDIUM, THE IMPROVEMENT WHICH COMPRISES: BRINGING A POROUS SOLIDABSORBENT MEMBER, ADAPTED TO ABSORB LIQUID, INTO PHYSICAL CONTACT WITHTHE FILM OF LIQUID ON THE SURFACE OF THE HIGH RESISTANCE MATERIALBEARING THE IMPERMANENT VISUA IMAGE CONSISTING OF ELECTROSTATICALLY-HELDSOLID PARTICLES AND THE FILM OF LIQUID ASSOCIATED THEREWITH, THEREBYABSORBING A SUBSTANTIAL PORTION OF THE LIQUID AND A MINOR PROPORTION OFTHE SOLID PARTICLES WHILE LEAVING A SUBSTANTIALLY LIQUID-FREE IMAGEFORMED OF THE REMAINDER OF THE SOLID PARTICLES AND THEREAFTERTRANSFERRING THE LIQUID-FREE IMAGE FORMED OF SOLID PARTICLES TO ANON-ABSORBENT RECORD MEDIUM.